It is common to drive a series string of LEDs using a positive voltage boost converter, where the LEDs are connected between the boosted output voltage terminal and ground. FIG. 1 illustrates such a conventional boost LED driver. The input voltage Vin, generated by a power supply PS, is coupled to one end of an inductor L. A boost controller 10 receives feedback signals and controls the duty cycle of transistor switches 12 and 14 to supply a regulated current through the string of LEDs 16. A MOSFET 18, connected in series with the LEDs 16, is controlled by a pulse-width-modulation (PWM) dimmer circuit to control the perceived brightness of the LEDs 16.
When the switch 14 is on, an upward ramping current flows through the inductor L to charge the inductor L to a regulated peak current. After the peak current is reached, the switch 14 is turned off and the switch 12 is turned on. A downward ramping inductor current flows through the switch 12. The switch 14 then turns back on at the beginning of the next switching cycle, controlled by an oscillator. The switch current is smoothed by the output capacitor Cout.
The switch current Isw and LED current ILED through the low value sense resistors 20 and 22 are sensed, by measuring the respective voltage drops across the resistors, to provide the Isw and ILED feedback signals to the boost controller 10. The controller 10 uses these feedback signals to control the switch 14 duty cycle to supply a target regulated current through the LEDs 16 when the MOSFET 18 is on.
The average current through the switch 12 is also the average current through the LEDs 16 and the sense resistor 22. The boost converter of FIG. 1 regulates the LED current to a target value, and the boosted voltage Vout across the LED string and the capacitor Cout is higher than the input voltage Vin.
There are other possible boost converter configurations.
In some instances, it is not desirable to use such a boost converter, such as if the required boosted output voltage is higher (relative to ground) than a level that is safe for an application. Additionally, other available switching controllers intended for non-boost topologies may have features that are desirable, but these features are not available in an available boost controller. For example, there may be features of a buck controller IC that are particularly appealing to a user wanting to drive a string of LEDs in a certain application, but the application requires a boosted voltage across the LEDs.
One possible way to drive a string of LEDs with a boosted voltage, while using a conventional buck controller IC, is shown in FIG. 2. FIG. 2 illustrates a positive-to-negative buck-boost converter using a conventional buck controller IC, renamed as a positive-to-negative buck-boost controller 24. In this topology, the anode end of the string of LEDs 16 is connected to ground, and the cathode end is connected to a negative voltage output terminal generating −Vee. Thus, the converter is a positive-to-negative converter, relative to ground.
When the highside switch 26 is on, an upward ramping current flows through the switch 26 and inductor L until a regulated peak current is reached. This highside switch control is common in buck converters.
After the peak current is reached, the switch 26 is turned off and the lowside switch 28 is turned on. The left end of the inductor L then goes negative and the inductor current IL ramps down. The switch 26 then turns back on at the beginning of the next switching cycle, determined by an oscillator. The inductor current IL and LED current ILED is sensed by the low value sense resistors 20 and 22 and provide feedback signals into the controller 24 for regulating the current through the LEDs 16 to match a target current set by the user. The output capacitor Cout smooths the ripple in the inductor current IL provided to the output.
In this topology, the average current through the inductor is the sum of the average current through the LEDs 16 plus the average input current that flows through the power supply PS via the ground terminal connected to the string of LEDs. As a result, the average current through the inductor L and the switches 26/28 is higher than the average current in the boost converter of FIG. 1.
This higher current results in significant power losses through the switches 26/28 and inductor L. Therefore, although the basic positive-to-negative buck-boost converter topology of FIG. 2 (using a conventional buck controller) offers an alternative to the boost converter of FIG. 1, it is less efficient than the boost converter.
Two advantages of the converter of FIG. 2, however, are low output ripple, which is common to a buck converter, and the use of a buck controller IC which may have a useful feature that is not found in an available boost controller IC, such as short-circuit protection. The boost converter of FIG. 1 typically has low input ripple (ripple on the power supply bus), since the power supply is placed in series with the inductor. The positive-to-negative buck-boost converter of FIG. 2 transfers a low input ripple to the output and transfers a higher ripple to the input (via the ground terminal), resulting in an undesirable high input ripple. This is common for a buck converter. This not only makes it harder to achieve a constant output current, but it causes ripple on the power supply bus which may affect other circuitry in the system.
What is needed is an LED driver that can use a conventional buck controller but does not suffer from the higher inefficiency and the high input ripple of the topology of FIG. 2.